The present invention relates to motor protection circuits and more particularly to a motor protection circuit for sensing conditions on both the line side and the motor side of the main motor contactors of the circuit.
Available motor protection circuits detect various fault conditions in multi-phase circuits such as low voltage, over voltage, phase shift or reversal, loss of a phase, ground faults and related problems. Upon detection of any one or more of the aforesaid conditions, the protection circuit opens the motor contactors to protect the motor from a faulty line condition or protect the line from a faulty motor.
In such circuits known to the prior art, sensing is wholly on the line side of the motor contactors so that actual conditions at the motor are often not detected or detected only after a destructive failure. Failures such as a broken wire between the motor side of a contactor and the motor cannot be detected until operation of the motor on two phases (or one phase in a two phase system) has damaged the motor. A similar result can occur from a broken or burned out contactor. A low voltage condition can also exist from a partly burned contactor or damaged lead or bus.
As indicated, the prior art fails to provide sensing on the motor side of the motor contactors; the dangers of such failure not being recognized.